This invention relates generally to devices for monitoring the level of liquid contained in a storage tank or the like, and more particularly to indicator devices of the type which employ an immersible probe assembly to sense changes in the conductive or capacitive characteristics thereof as the probe assembly becomes submerged in a rising level of fluid.
A number of prior gauging mechanisms have employed elongate upright probes which extended a substantial distance vertically along one side of a tank. Most devices measured either changes in conductance or changes in capacitance between the probes as the liquid level rose or fell, in order to provide a determination of the quantity of liquid contained in the tank. Connected to these probes were leads which extended to detector and indicator circuitry, the latter often being remotely located from the tank. One of the problems associated with such systems was that the appreciable lead length tended to pick up stray signals and hum, some of which were of sufficient magnitude so as to interfere with the conductance or capacitance measurements associated with the probe. Shielded leads were expensive and added large capacitance to the system, tending to mask the relatively small capacitance values which were being sensed by the probe.
Where the circuitry was housed inside the tank, there was frequently a danger of damage thereto due to moisture or leakage. Units which were satisfactorily sealed did not lend themselves to required periodic maintenance, testing or repair. As a result, the flexibility of such systems as well as their performance over long periods of time tended to suffer.